1. Field of the Invention
The present invention relates to a solder paste, and to a structure for electrically connecting two objects by interposing a solder layer between their terminals.
2. Description of the Related Art
Soldering is employed, for example, in mounting of electronic components to a circuit board. With today's electronic devices, surface mounting technology is used to achieve higher mounting density of parts on a circuit board. In this surface mounting of parts, so-called “solder paste” is used. The solder paste is typically a mixture of a solder alloy powder with rosin, an organic solvent, a viscosity regulator, and a surfactant.
Sn—Pb alloys have long been widely used as solder alloys. Because of the toxicity of lead (Pb), however, Pb-free solder alloys have increasing replaced the Pb-containing solder alloys. Some of the Pb-free solder alloys commonly used are Sn—Ag—Cu alloys and Sn—Zn alloys.
A typical example of the composition of a Sn—Ag—Cu alloy is Sn-3.0Ag-0.5Cu. Sn-3.0Ag-0.5Cu contains 96.5 examples of the composition of a Sn—Zn alloy are Sn-9Zn and Sn-8Zn-3Bi. Sn-9Zn contains 91 wt % tin and 9 wt % zinc, while Sn-8Zn-3Bi contains 89 wt % tin, 8 wt % zinc, and 3 wt % bismuth.
However, a Sn—Ag—Cu solder alloy has a higher melting point than a Sn—Pb solder alloy. For instance, the melting point of a Sn-3.0Ag-0.5Cu solder alloy is 218° C., which is 35° C. higher than the melting point of a typical conventional Sn—Pb solder alloy. Consequently, there is inevitably the danger that low heat-resistant parts may be damaged during soldering. Something must be done to avoid this situation, such as arranging poor a heat-resistant part near a large thermal-capacity part in the design of a printed circuit board. Furthermore, when a Sn—Ag—Cu solder alloy is used, the soldering temperature has to be higher than with a conventional solder, so the equipment used for soldering also has to be changed.
In contrast, a Sn—Zn solder alloy has a lower melting point. For example, a Sn-8Zn-3Bi solder alloy has a melting point of 193° C., whereas a Sn-9Zn solder alloy has a melting point of 199° C. In either case, the soldering temperature for the Sn—Zn solder alloy is only about 10° C. higher than the conventional soldering temperature. Consequently, there is no problem in terms of the heat resistance of parts, and soldering can be performed using existing equipment.
Nevertheless, when a Sn—Zn solder alloy is used, the connection strength may decrease over time until a break occurs at the connection, causing the soldered part to come loose from the circuit board.
This problem occurs when an electronic component is mounted using an Sn—Zn solder alloy over a copper wiring pattern formed on a circuit board, for instance. Because Zn and Cu have a high affinity with each other, a ZnCu compound layer is readily formed in the copper wiring pattern at the interface with the solder alloy layer. The ZnCu compound layer thus formed grows over time due to solid phase diffusion of Zn from the solder alloy layer. The temperature of an electronic device often reaches about 80° C. during the operation of the device, and the thermal stress that accompanies this rise in temperature accelerates the solid phase diffusion of Zn. The strength of the soldered portions therefore decreases over time as the ZnCu compound layer grows.